In order for an SIR to work properly for a passenger who is not seated behind a steering wheel, there must be provided some means to restrain forward movement of the passenger's legs so that the passenger's body moves forward in a position close to that when the passenger is normally seated. This is especially critical when the vehicle passenger is acting in an illegal and unwise manner by not utilizing their belt restraint.
In most vehicles, the downward sloping lower portion of the instrument panel fulfills the knee bolster requirement. This knee bolster must be positioned in the correct area and also must have some ability to give in a front crash situation. Although utilization of the instrument panel as a knee bolster is acceptable from an operational standpoint, it brings about two substantial disadvantages. One disadvantage of utilizing the instrument panel as a knee bolster is that additional space is taken up in the vehicle, denying the front seat passenger leg room. To recover the lost leg room, the total vehicle must be enlarged, or trunk or engine space must be reduced. Secondly, utilization of the instrument panel for a bolster function requires a larger instrument panel than absolutely necessary and thereby adds weight to the vehicle, diminishing the vehicle's environmental efficiency.